Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy

Michael T. Vagnini, Michael W. Mara, Michael R. Harpham, Jier Huang, Megan L. Shelby, Lin X. Chen, Michael R Wasielewski

Research output: Contribution to journalArticle

19 Citations (Scopus)

Abstract

Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(iv) state of the Ir(iii)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor-chromophore-Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI- 1*PMI-Ir(iii) → NDI-PMI--Ir(iv) and NDI- 1*PMI-Ir(iii) → NDI--PMI+-Ir(iii) that each proceed with τ <5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI--PMI-Ir(iv). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(iii) to Ir(iv), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor-acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.

Original languageEnglish
Pages (from-to)3863-3873
Number of pages11
JournalChemical Science
Volume4
Issue number10
DOIs
Publication statusPublished - Oct 2013

Fingerprint

X ray absorption spectroscopy
Chromophores
Oxidation
Catalysts
Electrons
Water
Photoexcitation
Perylene
Ions
Electrochemical oxidation
Methylene Chloride
X ray absorption
Absorption spectroscopy
Chemical activation
Spectroscopy

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy. / Vagnini, Michael T.; Mara, Michael W.; Harpham, Michael R.; Huang, Jier; Shelby, Megan L.; Chen, Lin X.; Wasielewski, Michael R.

In: Chemical Science, Vol. 4, No. 10, 10.2013, p. 3863-3873.

Research output: Contribution to journalArticle

Vagnini, Michael T. ; Mara, Michael W. ; Harpham, Michael R. ; Huang, Jier ; Shelby, Megan L. ; Chen, Lin X. ; Wasielewski, Michael R. / Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy. In: Chemical Science. 2013 ; Vol. 4, No. 10. pp. 3863-3873.
@article{6209175e45224fe3b3d057dbf18ac914,
title = "Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy",
abstract = "Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(iv) state of the Ir(iii)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor-chromophore-Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI- 1*PMI-Ir(iii) → NDI-PMI--Ir(iv) and NDI- 1*PMI-Ir(iii) → NDI--PMI+-Ir(iii) that each proceed with τ <5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI--PMI-Ir(iv). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(iii) to Ir(iv), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor-acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.",
author = "Vagnini, {Michael T.} and Mara, {Michael W.} and Harpham, {Michael R.} and Jier Huang and Shelby, {Megan L.} and Chen, {Lin X.} and Wasielewski, {Michael R}",
year = "2013",
month = "10",
doi = "10.1039/c3sc51511g",
language = "English",
volume = "4",
pages = "3863--3873",
journal = "Chemical Science",
issn = "2041-6520",
publisher = "Royal Society of Chemistry",
number = "10",

}

TY - JOUR

T1 - Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy

AU - Vagnini, Michael T.

AU - Mara, Michael W.

AU - Harpham, Michael R.

AU - Huang, Jier

AU - Shelby, Megan L.

AU - Chen, Lin X.

AU - Wasielewski, Michael R

PY - 2013/10

Y1 - 2013/10

N2 - Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(iv) state of the Ir(iii)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor-chromophore-Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI- 1*PMI-Ir(iii) → NDI-PMI--Ir(iv) and NDI- 1*PMI-Ir(iii) → NDI--PMI+-Ir(iii) that each proceed with τ <5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI--PMI-Ir(iv). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(iii) to Ir(iv), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor-acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.

AB - Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(iv) state of the Ir(iii)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor-chromophore-Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI- 1*PMI-Ir(iii) → NDI-PMI--Ir(iv) and NDI- 1*PMI-Ir(iii) → NDI--PMI+-Ir(iii) that each proceed with τ <5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI--PMI-Ir(iv). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(iii) to Ir(iv), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor-acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.

UR - http://www.scopus.com/inward/record.url?scp=84883264156&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=84883264156&partnerID=8YFLogxK

U2 - 10.1039/c3sc51511g

DO - 10.1039/c3sc51511g

M3 - Article

AN - SCOPUS:84883264156

VL - 4

SP - 3863

EP - 3873

JO - Chemical Science

JF - Chemical Science

SN - 2041-6520

IS - 10

ER -